1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a technology for alignment-treating the alignment layers of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device includes a pair of spaced and opposed substrates, an electrode and an alignment layer formed on one substrate, an electrode and an alignment layer formed on the other substrate, and a liquid crystal held between the pair of substrates. The electrode of one substrate is formed as a common electrode, and the electrode of the other substrate is formed as pixel electrodes. The pixel electrodes can be formed together with an active matrix. Either one of the substrates is provided with a black matrix and a color filter.
When the liquid crystal display device is viewed, the portions of pixel electrodes become bright and dark depending upon ˜h˜ presence and absence of a voltage. That is, the pixel electrodes define the pixel display portions. The openings of the black matrix are arranged to overlap the pixel electrodes, and have areas smaller than the areas of the pixel electrodes. When the black matrix is provided, therefore, the pixel display portions are defined by the openings of the black matrix. In either case, the portions around the pixel display portions are non-display portions.
The alignment layers can be grouped into horizontal alignment layers and vertical alignment layers. When the horizontal alignment layers are used, the liquid crystal molecules are oriented nearly parallel to the surface of the substrate. Upon applying a voltage, therefore, the liquid crystal molecules are tilted with respect to the surface of the substrate. When the vertical alignment layers are used, the liquid crystal molecules are oriented nearly perpendicularly to the surface of the substrate. Upon applying a voltage, therefore, the liquid crystal molecules are tilted obliquely to the surface of the substrate. In either case, the alignment layers are alignment-treated by a process such as being rubbed. In the case of a TN-type liquid crystal display device, the liquid crystal twists from one alignment layer toward the other alignment layer. Upon effecting the rubbing, furthermore, the liquid crystal molecules are in a pretilt position with a pretilt angle with respect to the alignment layer.
In the TN-type liquid crystal display device, as viewed in its assembled state, the two alignment layers are rubbed in directions forming a predetermined angle (90 degrees) relative to each other, so that the liquid crystal twists from one alignment layer toward the other alignment layer. Here, when it is regarded that the liquid crystal molecules are oriented in a plane, by neglecting the twist, the pretilt direction of the liquid crystal molecules positioned near one alignment layer is the same as the pretilt direction of the liquid crystal molecules positioned near the other alignment layer, whereby the liquid crystal molecules positioned between the alignment layers are oriented according to the pretilt directions of liquid crystal molecules positioned near the two alignment layers.
If the pretilt direction of the liquid crystal molecules positioned near one alignment layer is opposite to the pretilt direction of the liquid crystal molecules positioned near the other alignment layer, the intermediately positioned liquid crystal molecules may not be tilted in a predetermined direction because it is not certain that the liquid crystal molecules depend on any of the pretilt directions of liquid crystal molecules positioned near these alignment layers. The state of alignment becomes a spray-alignment when the pretilt angle of the alignment layer is close to the horizontal alignment, and becomes a bend-alignment when the pretilt angle of the alignment layer is higher than, for example, 30° or is close to the vertical alignment.
However, the assignee as for the present case has proposed the fact that even when the pretilt direction of the liquid crystal molecules positioned near one alignment layer is opposite to the pretilt direction of the liquid crystal molecules positioned near the other alignment layer, if there is a difference between the pretilt angle of the liquid crystal molecules near one alignment layer and the pretilt angle of the liquid crystal molecules near the other alignment layer, the intermediate liquid crystal molecules are oriented depending on the pretilt direction of the liquid crystal molecules having a larger pretilt angle (or a smaller pretilt angle). The assignee has utilized this fact in the alignment division that will be described below. When the horizontal alignment layers are used, the intermediate liquid crystal molecules are oriented depending on the pretilt direction of the liquid crystal molecules having a larger pretilt angle. When the vertical alignment layers are used, the intermediately positioned liquid crystal molecules are oriented depending on the pretilt direction of the liquid crystal molecules having a smaller pretilt angle.
The liquid crystal display device involves a problem of a so-called visual angle characteristic in which the display device appears whitish or blackish depending upon the direction from which the display surface is viewed. Technology for dividing alignment has been proposed for improving the problem of the visual angle characteristic. The alignment division consists of dividing a pixel into two domains, so that the liquid crystal molecules positioned between the two alignment layers in one domain are tilted in one direction and the liquid crystal molecules positioned between the two alignment layers in the other domain are tilted in the other direction. A whitish appearance and a blackish appearance are averaged by the alignment division, and a favorable display is obtained irrespective of the direction from which the display surface is viewed. To effect the alignment division, however, the alignment layer must be rubbed for each domain, and the alignment layers must be rubbed twice using a mask.
U.S. Pat. No. 5,473,455, assigned to the assignee of the present case, discloses various technologies for the alignment division. A particularly advantageous technology consists of effecting the alignment division by rubbing each alignment layer once. According to this technology for the alignment division, each alignment layer is rubbed and irradiated with ultraviolet rays via a mask. In the portions that are not irradiated with ultraviolet rays, the liquid crystal molecules are oriented at a first pretilt angle due to the effect of rubbing. In the portions irradiated with ultraviolet rays, the liquid crystal molecules are oriented at a second pretilt angle which is greater (or smaller) thus the first pretilt angle. A portion of one alignment layer having the first pretilt angle is arranged to be opposed to a portion of the other alignment layer having the second pretilt angle. The rubbing of the two alignment layers forms a bend-alignment or a spray-alignment. However, the intermediately positioned liquid crystal molecules are oriented according to the pretilt direction of the liquid crystal molecules having a particular pretilt angle since there exists a difference between the pretilt angles of the liquid crystal molecules near the two alignment layers. The solid portion of the mask and the opening appear alternatingly, and the directions of alignment of the liquid crystal alternate correspondingly.
The liquid crystal display device has pixel display portions and non-display portions. Here, the pixel display portions and the non-display portions are rubbed simultaneously. However, Japanese Unexamined Patent Publication (Kokai) No. 8-152638 proposes an alignment treatment in which the anchoring energy for the non-display portions is larger than the anchoring energy for the pixel display portions. According to this publication, in a state where a predetermined voltage is applied, the liquid crystal molecules are easily erected in the pixel display portions but the liquid crystal molecules are less easily erected in the non-display portions and are not affected by a voltage of a bus line applied to the non-display portions.
There exists a further problem in rubbing. The rubbing consists of rubbing the alignment layer with a cloth made of, for example, rayon, but dirt is produced as the cloth is brought into a clean room. Besides, the rubbing generates static electricity which may destroy TFTs (thin-film transistors) in the active matrix. It has therefore been desired to execute the alignment treatment by, for example, irradiation with ultraviolet rays instead of rubbing.
U.S. Pat. No. 4,974,941 and Japanese Unexamined Patent Publications (Kokai) No. 6-289374 and No. 8-015681, for example, disclose an alignment treatment using polarized ultraviolet rays. According to U.S. Pat. No. 4,974,941, a homogeneously oriented liquid crystal cell is partly irradiated with polarized ultraviolet rays so that the direction of alignment of the irradiated portion becomes different from the initial direction of homogeneous alignment. According to Japanese Unexamined Patent Publications (Kokai) No. 6-289374 and No. 8-015681, a mesh-like polymer tissue (PPN) that can be optically oriented is irradiated with perpendicularly polarized ultraviolet rays in order to realize the alignment of the liquid crystal molecules. With this method, however, a problem exists in that polarized ultraviolet rays must be used. A polarizer of the Glen-Taylor type is available for obtaining the polarized ultraviolet rays. However, the Glen-Taylor type polarizer is obtained by cutting a natural calcite and is not suited for practical use. It has therefore been desired to effect the alignment treatment by using non-polarized ultraviolet rays.
When the pixel display portions of the opposing alignment layers are differently oriented, the electric charge remains in large amounts near one alignment layer when a voltage is applied to a given pixel, while the liquid crystal display device is being used, and is then no longer applied. Even in a state where the voltage is no longer applied, the image that was previously displayed remains slightly visible due to the residual electric charge. When the alignment is divided by using ultraviolet rays, in particular, the one alignment layer includes a portion irradiated with ultraviolet rays and the other opposing alignment layer includes a portion that is not irradiated with ultraviolet rays. Therefore, a difference develops in the alignment treatment between the opposing portions of the two alignment layers, and the electric charge tends to remain in large amounts near one of the alignment layers.